The world has huge hydrocarbon reserves in the form of heavy oil. As used herein, the term “heavy oil” generally refers to bitumen, extra heavy oil, heavy oil or residual hydrocarbons, both natural and pyrogenous. Industry defines light crude oil as having an API gravity higher than 31.1° and lower than 870 kg/m3 density, medium oil as having an API gravity between 31.1° and 22.3° and having a density between 870 kg/m3 to 920 kg/m3, heavy oil as having an API gravity between 22.3° and 10° and a density between 920 kg/m3 to 1,000 kg/m3, and extra heavy oil as having an API gravity of less than 10° and a density higher than 1,000 kg/m3. In Canada, bitumen generally refers to extra heavy oil extracted from oil sands. Bitumen does not readily flow without being heated or diluted with low viscosity hydrocarbons.
The development of heavy oil reserves has been restricted by the poor transportability of heavy oil due to its extremely high viscosity components, and its poor processability due to foulants, coke precursors and catalyst poisoning components. These problematic components are collectively referred to herein as “contaminants”. The main contaminants are asphaltenic hydrocarbons and very high boiling point polyaromatic hydrocarbons.
In order to produce transportable and readily processable petroleum products suitable for conventional refining, it is necessary to remove the asphaltenic contaminants from the heavy oil. It is known to partially achieve this result by a series of conventional processes. For example, a wellhead emulsion can be processed by de-watering, thermal and chemical de-emulsification, settling, dehydration, cooling, diluent addition (for transportation), atmospheric and vacuum distillations, pentane deasphalting, following by propane deasphalting, and yet the recovered asphaltic material are not pure asphaltenes.
Asphaltic material generally refers to a residual liquid fraction of crude oil, and may include asphaltenes, resins and residual oil. Asphaltenes are complex molecules believed to consist of associated systems of polyaromatic sheets bearing alkyl side chains. They are often the heaviest and most polar fractions found in heavy oil. Heteroatoms O, N and S as well as metals V, Ni and Fe are also present in asphaltenes. The exact molecular structure of asphaltenes is not known because of the complexity of the asphaltene molecules. Therefore, the definitions of asphaltenes are based on their solubility. Generally, asphaltenes are the fraction of oil that is insoluble in paraffinic solvents such as n-heptane or n-pentane, and soluble in aromatic solvents such as benzene or toluene.
It is well known that asphaltenes can be separated from bitumen or asphaltenic crude oil by precipitation with paraffinic solvents such as pentane or heptane. It is conventionally believed that a high solvent to oil ratio is required to separate pure asphaltenes, in the order of 40:1 by volume. At lower solvent levels, commonly used in solvent deasphalting, substantial non-asphaltenic material will precipitate with the asphaltenes. Furthermore, solvent deasphalting relies on multiple theoretical stages of separation of barely immiscible hydrocarbon liquids, and cannot tolerate the presence of water.
The oil yield of solvent deasphalting is limited by the high viscosity of resultant asphaltic materials, particularly for high viscosity bitumen feed. Furthermore, it is difficult to achieve high quality oil with high oil yield, due to the difficulties in achieving clean separation of oil and asphaltic fractions.
In solvent deasphalting, asphalt (essentially asphaltene with residual oil) is produced as a very viscous hot liquid, which forms glassy solids when cooled. This viscous liquid must be heated to a high temperature in order to be transportable, which leads to fouling and plugging limitations.
Another technique for removal of asphaltenes involves breaking a froth of extra heavy oil and water with heat and a diluent solvent such as naphtha. In the case of paraffinic naphtha, partial asphaltene removal results. However, only about 50% of the asphaltenes may be readily removed with this treatment even with multiple stages, therefore, complete asphaltene removal is not practical. As a result, the resulting oils must still be processed by capital intensive technology which is relatively tolerant to asphaltenes.
Therefore, there is a need in the art for a method of selectively and efficiently removing asphaltenic contaminants from heavy oil, which mitigates the difficulties of the prior art.